Various types of engineered wood composites (e.g., oriented strand board, parallel strand lumber, laminated veneer lumber, and equivalents) are known and used in the construction of commercial and residential structures. Common applications for such wood composites include roof sheathing, wall sheathing, columns, flooring, structural insulated panels, engineered wood components (e.g. I-joists), cabinetry, and furniture. Most of these applications involve the risk of exposure to moisture. When a wood composite absorbs water, it will undergo dimensional change that is generally only partially reversible. Most wood composites are anisotropic with respect to their potential for dimensional change. Upon absorption of water, most wood composites will undergo greater expansion in the thickness dimension (on a percentage basis) than they will in the length or width dimensions. Depending on the moisture exposure conditions and duration, the increase in dimension might be predominantly localized, or alternatively, it might be essentially uniform. In either case, a dimensional change usually makes the wood composite more difficult to assemble or incorporate into a structure such as the floor, wall or roof of a building. Alternatively, wood composites may remain dry before assembly and subsequently encounter a condition which exposes the structure to water. Under these conditions, the wood composite parts may undergo dimensional changes that can result in problems such as buckling at joints or edge swell along seams in a roof, wall or floor system.
In addition to the problems discussed above, water absorption causes other problems in wood composites. For example, wood composites may undergo a loss of mechanical strength when subjected to water. Biological degradation may also be an issue when wood composites are hydrated to a threshold moisture content or higher. In general, exposure to water can substantially slow down the building process of a structure involving wood composites or compromise the durability and functionality of an existing structure.
The industry has long recognized the problems associated with water absorption in wood composites and has taken steps to inhibit the occurrence of water absorption and/or its effects. One approach is to incorporate a water-repelling agent during the process of producing the wood composite. Wood-based composites are generally made using a process similar to the schematic illustrated in FIG. 1. Referring to FIG. 1, wooden logs are cut into smaller wooden elements as depicted by the schematic step 102. A blender or other mixing device is used to apply a binder (e.g., a resin) and a water-repelling agent to the wooden elements as depicted in schematic step 106. The wooden elements are formed into a mat as shown in schematic step 108 and the mat is consolidated under heat and pressure as shown in schematic step 110. In most processes the wooden elements are subjected to a drying step (e.g., schematic step 104) at some point prior to the consolidation (schematic step 108). Additional processing steps may optionally be performed prior to the consolidation (schematic step 108) such as hydrating the wooden logs prior to refining; and screening and fractionating the wooden elements into different size classes prior to treatment with the binder and water-repelling agent. Additional procedures may optionally be performed after consolidation (schematic step 108) such as trimming and sawing the consolidated product into smaller pieces (e.g., schematic step 112), post-cooling, post-heating, grading, sorting, sanding, marking, labeling, stacking, sealing, packaging, and transporting (e.g., schematic step 114).
In general, two major categories of water-repellants have been used in the production of wood composites: wax emulsions and neat wax. By far, the most common wax products that have been used in the production of wood composites over the past fifty years are aqueous slack wax emulsions and neat (non-emulsified and non-aqueous) slack wax.
In North America, “slack wax” is produced by refining petroleum-derived lubricating oils, which are comprised of petroleum-derived hydrocarbon mixtures. Individual hydrocarbons in these mixtures that are liquids at standard temperature and pressure are categorized as “oils.” Slack waxes can be refined to have an oil-content as broad as 3% to 50%, but slack wax products used as water-repelling agents for wood composites commonly have an oil content of about 5% to 20%. The number of carbon atoms per molecule for the solid compounds in slack wax is known to be in the range of about 18 to about 48, but most of these compounds are in the range of about 36 to about 45 carbon atoms per molecule. The average molecular weight of the solid compounds of the wax component in slack wax in North America is about 500 to about 700 Daltons. The melt point of slack wax is dependent upon both the average molecular weight and the oil content. Slack waxes produced in North America commonly have a melt point of about 55° C. to about 70° C.
As an alternative to slack wax, “paraffin wax” has been considered for use in the production of wood composites. Like slack wax, paraffin is commercially available as a wax emulsion or neat wax form. Paraffin wax is also manufactured by refining petroleum-derived lubricating oils. Compared to slack wax, paraffin wax has a lower oil content (0% to 1%) and the solid compounds have a lower average molecular weight (360 to about 500 Daltons). Paraffin wax has a molecular distribution that predominantly ranges between about 20 and about 38 carbon atoms per molecule, such that the melt point ranges between about 48° C. to about 58° C.
In North America, there is a strong preference for slack wax as a water-repellant for wood composites. The preference is due to the fact that slack wax is readily available at a relatively low price; it is safe to use; and it can be converted into a low-viscosity liquid, either by melting or by emulsification in an aqueous medium. Notably, incorporation of molten slack wax or emulsified slack wax into a wood composite significantly reduces the rate at which the composite absorbs liquid water. Manufacturers of wood composites generally prefer to use emulsified slack wax over neat molten slack wax because the emulsified slack wax does not require a heated storage, transfer and application system. Other wax products have been contemplated as alternatives, but upon evaluation were found to be less desirable than slack wax due to inferior performance and/or unfavorable economics.
For example, in one experiment, particleboard was made with three different types of anionic, aqueous hydrocarbon emulsions. Roffael, E. & May, H., -A., “Paraffin Sizing of Particleboards: Chemical Aspects”, in Proceedings of the Seventeenth Washington State University International Particleboard/Composite Materials Series, (1983) ed. Maloney, T. M., p 283-295. The hydrocarbons used in these emulsions were reported to be either C20 paraffin, C28 paraffin or C36 paraffin. Id. Thus, these emulsions were quite unique in that the hydrocarbon components were not mixtures. Particleboards made with these emulsified water-repelling agents reportedly exhibited slower rates of water absorption and swelling as the size of the paraffin component in the emulsion increased. Id.
In a second part of the same study, particleboard was made with two different “commercially practical paraffins”, which were referred to as type I and type II. Id. The type I wax was reportedly comprised predominantly of C18 to C42 and had an oil content of 1.5% to 2.0% and a melt point of 51° C. to 53° C. Id. The type II wax was reportedly comprised predominantly of C16 to C30 and had an oil-content of 4.0% to 5.0% and a melt point of 42° C. to 44° C. Id. Particleboard made with the type I wax at a 0.25% loading level reportedly exhibited slower rates of thickness swell than particleboard made with the type II wax at a 0.25% loading level. Id. It should be noted that it is somewhat unclear in the second part of this experiment as to whether or not the “commercially practical paraffins” were applied as neat molten liquids or in emulsified form.
This publication and others generally show that when petroleum-derived waxes are used as a water-repelling agent in a particleboard application, higher molecular weight waxes are more effective than lower molecular weight waxes. Because the molecular weight of slack wax is greater than that of paraffin wax, the logical inference is that slack wax should perform better than paraffin wax in situations involving the use of these materials as water-repelling agents for wood composites. In consideration of the fact that paraffin wax is more expensive than slack wax, there would be little reason for any manufacturer to actually use paraffin wax as long as slack wax is available.
One drawback is that high levels of slack wax may interfere with the bonding action between wood elements and thus reduce the strength of the product. This presents a dilemma when one is attempting to make a wood composite that may simultaneously repel water and may have adequate strength properties for the particular application. In addition, the price of slack wax has risen over the years. Although slack wax is generally less expensive than paraffin wax, wax components represent a significant cost in the wood composite. There has been a trend in which petroleum refiners have reduced their production of the “lube stocks” that are used to make wax so that they can increase the yield of more profitable gasoline components, such as benzene and toluene. This shift in the refining practice has resulted in periodic shortages of slack wax in North America.
Thus, there is a need in the industry for alternative formulations for water-repelling agents for use in wood composites, which do not substantially impair the strength of the wood composite. There is also a need to develop an alternative formulation which does not significantly increase the cost of production when compared to conventional methods. There is also a need to develop a process for making a wood composite that employs alternative water-repelling agents.